Fluorescence regulation of 4-aminobenzofluoran and its applications for Cu2+-selective fluorescent probe and bioimaging

Fluoran dyes, a large family of colourants, have been widely used as thermosensitive dyes in print industry by taking advantage of the thermo-controlled interconversions between the colorless spirolactone and the colored zwitterion forms. But few attentions have been paid to their fluorescence properties. In this paper, the fluorescence regulation of 4-aminobenzofluoran dye 1 by introducing of electron deficient acetyl group to tune the energy level of the highest occupied molecular orbital (HOMO) of the amino group has been carried out. Based on this strategy, the N-acetylated product BF derived from its precursor 1 shows stable UV Vis and fluorescence spectra in water from pH 1.0 to 12.0 due to the blocking of the photoinduced electron transfer (PET) process from the amino group to the excited state of fluoran chromophore. Moreover, BF is cell permeable and can be used as signal reporter for the design of fluorescence probes by taking advantage of the fluorescence controlled mechanism of the ring opening and closing chemical processes of the spirolactone platform. By using BF as precursor, a highly sensitive and selective fluorescent probe BF-Cu for Cu2+ was synthesized by introducing a Cu2+-responsive water soluble 2-hydroxylethyl hydrazine functional group. We expect that the fluorescence recovery of fluoran dyes via the regulation of the HOMO energy level of the amino group enable an additional synthetic effort on the development of fluorescence fluoran dyes and expansion of their applications. (C) 2017 Elsevier Ltd. All rights reserved.